In the near future, renewable energy sources such as wind energy and solar energy will be the main sources of power used in distributed generation platforms. Nowadays, renewable energy power conditioning systems primarily deliver active power to the utility grid, which means that the generated current is in phase with the grid voltage. However, the growing prevalence of renewable energy sources requires faster and more flexible active/reactive power control. Since renewable energy sources are intermittent in nature, fast active/reactive power control can compensate for deficiencies in the reliability of such energy sources. For example, if there is a fluctuation in the utility grid voltage, the immediate injection of reactive power can stabilize the voltage. Thus, for renewable energy sources to replace mainstream power generation and to become a reliable alternative, renewable energy power conditioning systems must be able to provide rapid active/reactive power control.
In single-phase power conditioning systems, active/reactive power control is conventionally performed either in the stationary reference frame or in the synchronous reference frame (i.e. synchronous with the grid voltage angle). FIG. 1 shows a control system in the stationary reference frame while FIG. 2 shows a control system in the synchronous (rotating) reference frame. In FIG. 1, the control variables are in the stationary reference frame and the control system consists of two cascaded loops. The first loop is a fast internal current control loop, which is responsible for regulating the converter output current (the grid current). This loop deals with power quality issues and current protection. The second loop is an external power control loop, which is responsible for regulating the active/reactive output power. In FIG. 2, the control variables are in the synchronous reference frame and the structure of the control system is similar to that of the stationary reference frame with one notable difference. Since the control variables are DC values, simple PI controllers can be used to regulate the output current. However, the necessity of having cross-coupling terms in the synchronous reference frame diminishes the precision of the control method.
The control system in the stationary reference frame and the control system in the synchronous reference frame both use the same method to determine whether active/reactive power is required. This method consists of measuring the average values of the active and reactive power for the external power control loop. In single-phase power conditioning systems, measuring the active/reactive power requires a low-pass filter with a low bandwidth in order to filter out the double frequency ripple inherent in the output power. This low-pass filter introduces a delay into the control system. This delay and its attendant slowness in reacting to conditions is the main drawback associated with conventional control systems as shown in FIG. 1 and FIG. 2. However, as renewable energy sources inevitably become more numerous and replace mainstream power generation, such energy sources must meet demands to provide smart grid functionality. Unfortunately, such functionality cannot be achieved with a sluggish control system. What is needed is a control system that is able to provide smart grid functionality using a fast and reliable closed-loop control system.
For clarity, it should be noted that smart grid functionality includes the following features: active voltage regulation, active power control, and fault ride-through capabilities. Active voltage regulation is implemented by controlling the reactive power injected into the utility grid in order to stabilize the grid voltage. Active power control is provided by controlling the active power injected into the utility grid in order to stabilize the grid frequency. Finally, fault ride-through is provided by allowing the renewable energy power conditioning system to remain connected to the utility grid and supply reactive power to support the utility grid voltage during fault/disturbance conditions. These features of smart grid functionality require rapid active/reactive power control. Thus, in order for a power conditioning control system to provide smart grid functionality, it must also be able to achieve fast active/reactive power control.